Many materials are used as dielectrics in capacitors. These include glasses, ceramics, non-conductive metal oxides, plastics and polymers, and minerals of various types. The disadvantage of these non-ferroelectric dielectrics are the low dielectric constant of most of these materials. Most of these materials have relative permittivity values in the range of 1 to 100. A low dielectric constant requires that a capacitor be physically large to obtain large capacitance values, so the primary disadvantage is the size and weight of capacitors made with these materials. High dielectric ceramics gain their properties from an instability of the crystalline lattice which causes the ceramic to change phases with temperature. In certain phases the instability results in large values of permittivity, but the permittivity changes markedly with temperature.
Ferroelectric ceramics have also been used as capacitor dielectrics. The disadvantage of ferroelectric capacitors for use in most circuit applications is hysteresis. The hysteresis creates two disadvantages: it lowers the effective permittivity, and it creates switching effects which are unwanted in most circuits. When a voltage is first applied to a ferroelectric capacitor a large amount of charge is absorbed. This makes it appear as if the capacitor has a large dielectric constant. However, when the applied voltage is refumed to zero, only a fraction of the charge that went into the capacitor flows back out. The bulk of the charge is stored more or less permanently as remanent polarization in the capacitor. The only way to remove the remanent polarization electrically is to apply a reverse bias of the same magnitude as applied originally, which causes a reverse switching of the remanent polarization. This hysteresis means that the true dielectric constant of the capacitor as used in the circuit is actually much lower than the value computed when the remanent charge is included.